1. Field of the Invention
The invention relates to fabricating semiconductor chips. More specifically, the invention relates to a method and an apparatus for increasing the maximum intensity of poorly-defined regions in an image formed by a complementary mask that is normally used to expose (and remove) unwanted regions located between phase shifters of a phase shifting mask during an optical lithography process used in fabricating a semiconductor chip.
2. Related Art
Optical lithography process begins with the formation of a photoresist layer on the surface of a semiconductor wafer. A mask composed of opaque regions, which are generally formed of chrome, and light-transmitting clear regions, which are generally formed of quartz, is then positioned over this photoresist layer. (Note that the term “mask” as used in this specification is meant to include the term “reticle.”) Light is then shone on the mask from a visible light source, an ultraviolet light source, or more generally some type of electromagnetic radiation source together with suitably adapted masks and lithography equipment.
This image is reduced and focused through an optical system containing a number of lenses, filters, and mirrors. The light passes through the clear regions of the mask and exposes the underlying photoresist layer. At the same time, opaque regions of the mask block the light leaving underlying portions of the photoresist layer unexposed.
The exposed photoresist layer is then developed, through chemical removal of either the exposed or non-exposed regions of the photoresist layer. The end result is a semiconductor wafer with a photoresist layer having a desired pattern. This pattern can then be used for etching underlying regions of the wafer.
As integration densities continue to increase, it is becoming desirable to use phase shifters to define more and more features within a layout. This can lead to problems in some situations. For example, a desired feature can be formed using zero-degree phase shifter and 180-degree phase shifter of a phase shifting mask (PSM), and normally there remains a small region between shifters which is not intended to print. Such regions on a wafer under fabrication which result from use of the phase shifting mask are normally cleared by a complementary, or trim, mask during the optical lithography process.
A complementary mask 101 shown in FIG. 1B (also known as a trim mask) for use with a phase shifting mask (not shown) provides a number of openings at appropriate locations to fully expose any unwanted photoresist that may be left from use of the phase shifting mask. For example, opening 101A on complementary mask 101 is located to remove unexposed photoresist that would cause bridging between features 102Q and 102R (see left side of FIG. 1B). Note that the features to be formed are shown in FIG. 1B superposed on complementary mask 101 for ease in illustrating the locations of openings and features relative to one another. Also note that although a limited number of openings are shown in the attached figures for illustrative purposes, it will be understood that any number of such openings may be present in a complementary mask and/or phase shifting mask, depending on, for example, circuitry in a wafer to be formed using the masks.
As technology dimensions shrink to 65 nm and smaller, the intensity of radiation through one or more of openings (also called “cuts” or “cutouts”) 101A–101H in trim mask 101 (FIG. 1A) drops to such a low level as to become ineffective in exposing (and therefore removing) unwanted photoresist. The issue of insufficient intensity through the trim mask openings imposes limits on the overall performance of full phase technology using such a trim mask (following the use of a phase shifting mask).
For an example of this problem, see aerial image 103 (FIG. 1C) illustrating the effects of openings 101A–101H in trim mask 101 (FIG. 1A). Image 103 was obtained from a trim-level test cell layout with 65 nm design rules and aerial image simulation at the following image settings: λ=193 nm, 0.8 NA, and 0.2 σ (partial coherence) at best focus. FIG. 1C shows a contour plot of the aerial image, with levels of intensity being marked by solid or dotted lines. Note that image 103 has its brightest intensity in region 104E (FIG. 1C) which is surrounded by a slightly lower intensity region 103E. Regions 103E and 104E are portions of a continuum of intensities with the highest intensity at the center of region 104E.
As shown in FIG. 1C, opening 101A (which may be, for example, a square of 10 nm on each side) produces a region of very low intensity, namely region 103D (FIG. 1C) which is shown by a dotted line. For this reason, region 103D will be “poorly defined” on a wafer. Due to unacceptably low intensity in several regions (e.g. regions 103A and 103D), their respective openings (e.g. openings 101A and 101D) are ineffective in exposing—and therefore permitting removal of—unwanted photoresist in conjunction with the use of a phase shifting mask. For example, when such a trim mask 101 (FIG. 1A) is used, features 102Q and 102R (FIG. 1B) are likely to remain connected to one another despite the presence of opening 101A, because of insufficient intensity in the corresponding regions of the aerial image generated by mask 101.
In some embodiments the complementary, or trim, mask may include phase shifters, e.g. attenuated background.